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Aircraft maintenance: Major vs. minor alterations

In the last article, we explored “mysterious modifications” that seem to regularly appear on aircraft and how to evaluate their legality. This time, let’s dive head-first into the deep end with a discussion about major vs. minor alterations and how to make some sense of the federal aviation regulations (FARs) when it comes time to modify an aircraft. This is a complex and often controversial topic, so we will have to cover it in parts. That said, if you stick with me, you will either gain great appreciation for the world your IA lives in, or at the very least, give you a headache trying.

Jeff SimonUnderstanding the FARs is a lot like a scavenger hunt; you start at one regulation, which inevitably references two other regulations. Those two regulations reference five more, and so on. And, as you piece together the big picture, you will often come across verses in the FARs which seem to be contradictory to one another. Some degree of interpretation is required with complex situations, and it doesn’t help that you may get different answers from different FAA representatives if you seek guidance (which is why many mechanics try to avoid “seeking guidance”). What may be acceptable to one flight standards district office inspector may not be acceptable to another. Recently, the FAA has made a concerted effort to provide clear guidance materials in order to reduce these variations. They’ve come far from where they were 10 years ago, but there is still a long way to go.

In an attempt to help their own, the FAA also created guidance materials such as the Airworthiness Inspector’s Handbook, and more recently, the Flight Standards Information System (FSIMS) to clarify some of the gray areas. 

Why major vs. minor?

The most important place to start with any repair or alteration is with an assessment of whether it is a major or minor alteration. This is critical because the approval and documentation requirements for major vs. minor alterations are significantly different and, in fact, can be the difference between a simple logbook entry and a complex FAA approval process.

Minor alterations require only two things:

  1. Data that is “acceptable” to the FAA; and
  2. A logbook entry.

Major alterations require:

  1. FAA “approved data”;
  2. Recording of an FAA Form 337; and
  3. A logbook entry.

If the alteration is minor, the approval can be done by any licensed A&P mechanic. However, if the alteration is deemed to be major, it requires a mechanic with an Inspection Authorization (IA), in order to sign the approval block on the 337 form and submit it to the FAA. Any STC is, by definition, a major alteration and also requires a 337 form.

It seems to be a common practice for some mechanics to file a 337 form “just in case” in order to avoid any interpretation issues, but I would strongly advise against that approach.  A 337 form is titled “Major Repair & Alteration” for a reason. By definition, the FAA should reject any 337 form that does not constitute a major alteration and they sometimes do. In addition, the act of passing the judgment on to the FAA actually weakens the authority of the A&P’s and IA’s in the field. But, most importantly to an owner, stuffing your logbooks full of unnecessary 337 forms can mislead a future buyer regarding the amount and depth of work that has been done to the aircraft in the past.

So, I would encourage every owner considering an alteration or improvement to sit down with his or her mechanic and review the paperwork that the FARs require before diving into the job.

This segment was Step 1 in the process. Next time, we’ll really get deep and talk about acceptable vs. approved data, and what you really can do to that aircraft of yours!

Social FlightJeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 10,000 aviation events. Free apps available for iPhone, iPad and Android, and on the Web at

Jeff Simon
Jeff Simon
Jeff Simon is an A&P mechanic, IA, pilot, and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first inspection tool for geared alternator couplings available at Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 20,000 aviation events, hundred-dollar hamburger destinations, and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit
Topics: Aircraft Maintenance, Ownership, Gear

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